Conventionally, as methods for fixing images to transfer paper or the like in the above-mentioned image-forming devices, fixing techniques employing heat, pressure, or a combination of both have been proposed. Examples thereof include the oven fixing method, flash fixing method, pressure fixing method, hot roller fixing method, and the like. Of these, the hot roller fixing method is extensively employed from the standpoints of thermal efficiency, the danger of taking fire, fixing properties, etc.
As shown in FIG. 4, the above hot roller fixing method is a technique in which a hot roller 18 and a press roller 20 are arranged in a vertical stack and a transfer paper 14 is passed between the rollers 18 and 20. In this technique, a heat-sensitive ink 9 as a substance to be fixed is fused and fixed to the transfer paper 14 by heating the ink with a built-in fixing heater 19 in the hot roller 18 and, at the same time, fixing of the ink is made tenacious by pressing the ink with the press roller 20, thereby making an image by the heat-sensitive ink 9 on the transfer paper 14. In this case, for the purpose of preventing the heat-sensitive ink 9 or the transfer paper 14 to which the ink has been fixed from adhering to the hot roller 18 after the fixing, coating of a fluorine compound on the surface of the hot roller 18 or coating of a silicone oil on the surface of the above coating layer is being conducted. Further, since the above treatments only is insufficient in preventing the adhesion of the heat-sensitive ink 9 or transfer paper 14 to the hot roller 18, a separating blade 21 is provided, in addition to the above treatment, in such a manner that the blade is in slightly contact with the surface of the hot roller 18 as shown in FIG. 5, to thereby prevent the heat-sensitive ink 9 or the transfer paper 14 to which the ink has been fixed from adhering to the hot roller 18. However, provision of the separating blade 21 in an image-forming device as described above makes the device expensive and also causes a problem that the complicated device mechanism leads to frequent occurrence of troubles.
On the other hand, in order to fix the heat-sensitive ink 9 to the transfer paper 14, the hot roller 18 should be heated to a temperature required for the fixing. However, if the temperature of the hot roller 18 is raised too high, there are cases where the heat-sensitive ink 9 cannot be completely peeled from the hot roller 18 because the heat-sensitive ink 9 itself has a slightly tacky nature. As a result, not only the quality of the image formed on the transfer paper 14 becomes insufficient, but the resistance at the time of peeling from the hot roller 18 causes problems that the transfer paper 14 is electrostatically charged due to the peeling, resulting in adhesion of dust particles to the charged transfer paper 14, and that the hot roller 18 and the separating blade 19 are electrostatically charged with static electricity.
In order to solve the problems as described above, a system is being studied in which a peeling roller (not shown), for example, is used as the third roller in addition to the hot roller 18 and the press roller 20, with the peeling roller being provided on a side of the hot roller 18, and a fixing belt made of a flexible composite tubular article comprising a fluoroplastic and a polyimide resin is stretched between the hot roller 18 and the peeling roller, and in which the transfer paper 14 printed with the heat-sensitive ink 9 is passed between the belt and the press roller 20. The fluoroplastic employed in the above fixing belt is excellent in release properties, chemical resistance, and heat resistance, and has conventionally been used in large quantities for release agents, sealing materials, and the like. The above-mentioned polyimide resin is excellent in heat resistance, chemical resistance, electrical insulating properties, mechanical properties, etc. Therefore, the objective of the adoption of the fixing belt made of such a composite tubular article is to simply and effectively attain improved transfer and release properties etc. by employing a fluoroplastic layer, having excellent heat resistance and release properties, as the outer layer and a polyimide resin layer, having excellent heat resistance, electrical insulating properties, and mechanical properties, as the inner layer in the fixing belt. A process for producing such a composite tubular article is described in JP-A-61-95361. (The term "JP-A" used herein means an unexamined published Japanese patent application.) In this production process, a solution of a precursor of polyimide resin is mixed with an electrically conductive fine powder to prepare a liquid mixture, and this mixture is placed in a large-diameter cylindrical vessel and subjected to centrifugal casting with heating, thereby to form a thin layer along the inner circumferential surface of the cylinder. An endless belt (having an electrically conductive layer) made of this thin layer is then formed, and on the surface of the electrically conductive layer of the belt, a fluoroplastic is spray-coated to form a dielectric layer, thereby preparing a photo-insensitive dielectric belt.